$Id$ TOR Spec Note: This is an attempt to specify TOR as it exists as implemented in early June, 2003. It is not recommended that others implement this design as it stands; future versions of TOR will implement improved protocols. 0. Notation: PK -- a public key. SK -- a private key K -- a key for a symmetric cypher a|b -- concatenation of 'a' with 'b'. a[i:j] -- Bytes 'i' through 'j'-1 (inclusive) of the string a. All numeric values are encoded in network (big-endian) order. Unless otherwise specified, all symmetric ciphers are 3DES in OFB mode, with an IV of all 0 bytes. Asymmetric ciphers are either RSA with 1024-bit keys and exponents of 65537, or DH with the safe prime from rfc2409, section 6.2, whose hex representation is: "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08" "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B" "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9" "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6" "49286651ECE65381FFFFFFFFFFFFFFFF" [We will move to AES once we can assume everybody will have it. -RD] 1. System overview Tor is a connection-oriented anonymizing communication service. Users build a path known as a "virtual circuit" through the network, in which each node knows its predecessor and successor, but no others. Traffic flowing down the circuit is unwrapped by a symmetric key at each node, which reveals the downstream node. 2. Connections 2.1. Establishing OR connections When one onion router opens a connection to another, the initiating OR (called the 'client') and the listening OR (called the 'server') perform the following handshake. [or when an op wants to connect to or] Before the handshake begins, the client and server know one another's (1024-bit) public keys, IPV4 addresses, and ports. 1. Client connects to server: The client generates a pair of 8-byte symmetric keys (one [K_f] for the 'forward' stream from client to server, and one [K_b] for the 'backward' stream from server to client. The client then generates a 'Client authentication' message [M] containing: The number 2 to signify OR handshake [2 bytes] The client's published IPV4 address [4 bytes] The client's published port [2 bytes] The server's published IPV4 address [4 bytes] The server's published port [2 bytes] The forward key (K_f) [16 bytes] The backward key (K_f) [16 bytes] The maximum bandwidth (bytes/s) [4 bytes] [Total: 50 bytes] The client then RSA-encrypts [M] with the server's public key and PKCS1 padding to give an encrypted message. The client then opens a TCP connection to the server, sends the 128-byte RSA-encrypted data to the server, and waits for a reply. 2. Server authenticates to client: Upon receiving a TCP connection, the server waits to receive 128 bytes from the client. It decrypts the message with its private key, and checks the PKCS1 padding. If the padding is incorrect, or if the message's length is other than 50 bytes, the server closes the TCP connection and stops handshaking. The server then checks the list of known ORs for one with the address and port given in the client's authentication. If no such OR is known, or if the server is already connected to that OR, the server closes the current TCP connection and stops handshaking. For later use, the server sets its keys for this connection, setting K_f to the client's K_b, and K_b to the client's K_f. The server then creates a server authentication message[M2] as follows: Modified client authentication [48 bytes] A random nonce [N] [8 bytes] [Total: 56 bytes] The client authentication is generated from M by replacing the client's preferred bandwidth [B_c] with the server's preferred bandwidth [B_s], if B_s < B_c. The server encrypts M2 with the client's public key (found from the list of known routers), using PKCS1 padding. The server sends the 128-byte encrypted message to the client, and waits for a reply. 3. Client authenticates to server. Once the client has received 128 bytes, it decrypts them with its public key, and checks the PKCS1 padding. If the padding is invalid, or the decrypted message's length is other than 56 bytes, the client closes the TCP connection. The client checks that the addresses and keys in the reply message are the same as the ones it originally sent. If not, it closes the TCP connection. The client updates the connection's bandwidth to that set by the server, and generates the following authentication message [M3]: The client's published IPV4 address [4 bytes] The client's published port [2 bytes] The server's published IPV4 address [4 bytes] The server's published port [2 bytes] The server-generated nonce [N] [8 bytes] [Total: 20 bytes] Once again, the client encrypts this message using the server's public key and PKCS1 padding, and sends the resulting 128-byte message to the server. 4. Server checks client authentication The server once again waits to receive 128 bytes from the client, decrypts the message with its private key, and checks the PKCS1 padding. If the padding is incorrect, or if the message's length is other than 20 bytes, the server closes the TCP connection and stops handshaking. If the addresses in the decrypted message M3 match those in M and M2, and if the nonce in M3 is the same as in M2, the handshake is complete, and the client and server begin sending cells to one another. Otherwise, the server closes the TCP connection. 2.2. Establishing OP-to-OR connections [wrap this with the above] When an Onion Proxy (OP) needs to establish a connection to an OR, the handshake is simpler because the OR does not need to verify the OP's identity. The OP and OR establish the following steps: 1. OP connects to OR: First, the OP generates a pair of 8-byte symmetric keys (one [K_f] for the 'forward' stream from OP to OR, and one [K_b] for the 'backward' stream from OR to OP). The OP generates a message [M] in the following format: The number 1 to signify OP handshake [2 bytes] Maximum bandwidth (bytes/s) [4 bytes] Forward key [K_f] [16 bytes] Backward key [K_b] [16 bytes] [Total: 38 bytes] The OP encrypts M with the OR's public key and PKCS1 padding, opens a TCP connection to the OR's TCP port, and sends the resulting 128-byte encrypted message to the OR. 2. OR receives keys: When the OR receives a connection from an OP [This is on a different port, right? How does it know the difference? -NM], [Correct. The 'or_port' config variable specifies the OR port, and the op_port variable specified the OP port. -RD] it waits for 128 bytes of data, and decrypts the resulting data with its private key, checking the PKCS1 padding. If the padding is invalid, or the message is not 38 bytes long, the OR closes the connection. Otherwise, the connection is established, and the OR is ready to receive cells. The server sets its keys for this connection, setting K_f to the client's K_b, and K_b to the client's K_f. 2.3. Sending cells and link encryption Once the handshake is complete, the ORs or OR and OP send cells (specified below) to one another. Cells are sent serially, encrypted with the 3DES-OFB keystream specified by the handshake protocol. Over a connection, communicants encrypt outgoing cells with the connection's K_f, and decrypt incoming cells with the connection's K_b. [Commentary: This means that OR/OP->OR connections are malleable; I can flip bits in cells as they go across the wire, and see flipped bits coming out the cells as they are decrypted at the next server. I need to look more at the data format to see whether this is exploitable, but if there's no integrity checking there either, I suspect we may have an attack here. -NM] [Yes, this protocol is open to tagging attacks. The payloads are encrypted inside the network, so it's only at the edge node and beyond that it's a worry. But adversaries can already count packets and observe/modify timing. It's not worth putting in hashes; indeed, it would be quite hard, because one of the sides of the circuit doesn't know the keys that are used for de/encrypting at each hop, so couldn't craft hashes anyway. See the Bandwidth Throttling (threat model) thread on http://archives.seul.org/or/dev/Jul-2002/threads.html. -RD] [Even if I don't control both sides of the connection, I can still do evil stuff. For instance, if I can guess that a cell is a TOPIC_COMMAND_BEGIN cell to www.slashdot.org:80 , I can change the address and port to point to a machine I control. -NM] 3. Cell Packet format The basic unit of communication for onion routers and onion proxies is a fixed-width "cell". Each cell contains the following fields: ACI (anonymous circuit identifier) [2 bytes] Command [1 byte] Length [1 byte] Sequence number (unused, set to 0) [4 bytes] Payload (padded with 0 bytes) [248 bytes] [Total size: 256 bytes] The 'Command' field holds one of the following values: 0 -- PADDING (Padding) (See Sec 6.2) 1 -- CREATE (Create a circuit) (See Sec 4) 2 -- CREATED (Acknowledge create) (See Sec 4) 3 -- RELAY (End-to-end data) (See Sec 5) 4 -- DESTROY (Stop using a circuit) (See Sec 4) The interpretation of 'Length' and 'Payload' depend on the type of the cell. PADDING: Neither field is used. CREATE: Length is 144; the payload contains the first phase of the DH handshake. CREATED: Length is 128; the payload contains the second phase of the DH handshake. RELAY: Length is a value between 8 and 248; the first 'length' bytes of payload contain useful data. DESTROY: Neither field is used. Unused fields are filled with 0 bytes. The payload is padded with 0 bytes. PADDING cells are currently used to implement connection keepalive. ORs and OPs send one another a PADDING cell every few minutes. CREATE and DESTROY cells are used to manage circuits; see section 4 below. RELAY cells are used to send commands and data along a circuit; see section 5 below. 4. Circuit management 4.1. CREATE and CREATED cells Users set up circuits incrementally, one hop at a time. To create a new circuit, users send a CREATE cell to the first node, with the first half of the DH handshake; that node responds with a CREATED cell with the second half of the DH handshake. To extend a circuit past the first hop, the user sends an EXTEND relay cell (see section 5) which instructs the last node in the circuit to send a CREATE cell to extend the circuit. The payload for a CREATE cell is an 'onion skin', consisting of: RSA-encrypted data [128 bytes] Symmetrically-encrypted data [16 bytes] The RSA-encrypted portion contains: Symmetric key [16 bytes] First part of DH data (g^x) [112 bytes] The symmetrically encrypted portion contains: Second part of DH data (g^x) [16 bytes] The two parts of the DH data, once decrypted and concatenated, form g^x as calculated by the client. The relay payload for an EXTEND relay cell consists of: Address [4 bytes] Port [2 bytes] Onion skin [144 bytes] The port and address field denote the IPV4 address and port of the next onion router in the circuit. 4.2. Setting circuit keys Once the handshake between the OP and an OR is completed, both servers can now calculate g^xy with ordinary DH. They divide the last 32 bytes of this shared secret into two 16-byte keys, the first of which (called Kf) is used to encrypt the stream of data going from the OP to the OR, and second of which (called Kb) is used to encrypt the stream of data going from the OR to the OP. 4.3. Creating circuits When creating a circuit through the network, the circuit creator performs the following steps: 1. Choose a chain of N onion routers (R_1...R_N) to constitute the path, such that no router appears in the path twice. 2. If not already connected to the first router in the chain, open a new connection to that router. 3. Choose an ACI not already in use on the connection with the first router in the chain. If our address/port pair is numerically higher than the address/port pair of the other side, then let the high bit of the ACI be 1, else 0. 4. Send a CREATE cell along the connection, to be received by the first onion router. 5. Wait until a CREATED cell is received; finish the handshake and extract the forward key Kf_1 and the back key Kb_1. 6. For each subsequent onion router R (R_2 through R_N), extend the circuit to R. To extend the circuit by a single onion router R_M, the circuit creator performs these steps: 1. Create an onion skin, encrypting the RSA-encrypted part with R's public key. 2. Encrypt and send the onion skin in a RELAY_CREATE cell along the circuit (see section 5). 3. When a RELAY_CREATED cell is received, calculate the shared keys. The circuit is now extended. Upon receiving a CREATE cell along a connection, an OR performs the following steps: 1. If we already have an 'open' circuit along this connection with this ACI, drop the cell. Otherwise, if we have no circuit along this connection with this ACI, let L = the integer value of the first 4 bytes of the payload. Create a half-open circuit with this ACI, and begin queueing CREATE cells for this circuit. Otherwise, we have a half-open circuit. If the total payload length of the CREATE cells for this circuit is exactly equal to the onion length specified in the first cell (minus 4), then process the onion. If it is more, then tear down the circuit. 2. Once we have a complete onion, decrypt the first 128 bytes of the onion with this OR's RSA private key, and extract the outmost onion layer. If the version, back cipher, or forward cipher is unrecognized, or the expiration time is in the past, then tear down the circuit (see section 4.2). Compute K1 through K3 as above. Use K1 to decrypt the rest of the onion using 3DES/OFB. If we are not the exit node, remove the first layer from the decrypted onion, and send the remainder to the next OR on the circuit, as specified above. (Note that we'll choose a different ACI for this circuit on the connection with the next OR.) When an onion router receives an EXTEND relay cell, it sends a CREATE cell to the next onion router, with the enclosed onion skin as its payload. The initiating onion router chooses some random ACI not yet used on the connection between the two onion routers. Some time after receiving a create cell, an onion router completes the DH handshake, and replies with a CREATED cell, containing g^y as its [128 byte] payload. Upon receiving a CREATED cell, an onion router packs it payload into an EXTENDED relay cell (see section 5), and sends that cell up the circuit. Upon receiving the EXTENDED relay cell, the OP can retrieve g^y. (As an optimization, OR implementations may delay processing onions until a break in traffic allows time to do so without harming network latency too greatly.) 4.2. Tearing down circuits [Note: this section is untouched; the code doesn't seem to match what I remembered discussing. Let's sort it out. -NM] Circuits are torn down when an unrecoverable error occurs along the circuit, or when all streams on a circuit are closed and the circuit's intended lifetime is over. To tear down a circuit, an OR or OP sends a DESTROY cell with that direction's ACI to the adjacent nodes on that circuit. Upon receiving a DESTROY cell, an OR frees resources associated with the corresponding circuit. If it's not the start or end of the circuit, it sends a DESTROY cell for that circuit to the next OR in the circuit. If the node is the start or end of the circuit, then it tears down any associated edge connections (see section 5.1). After a DESTROY cell has been processed, an OR ignores all data or destroy cells for the corresponding circuit. 4.3. Routing data cells When an OR receives a RELAY cell, it checks the cell's ACI and determines whether it has a corresponding circuit along that connection. If not, the OR drops the RELAY cell. Otherwise, if the OR is not at the OP edge of the circuit (that is, either an 'exit node' or a non-edge node), it de/encrypts the length field and the payload with 3DES/OFB, as follows: 'Forward' relay cell (same direction as CREATE): Use Kf as key; encrypt. 'Back' relay cell (opposite direction from CREATE): Use Kb as key; decrypt. If the OR recognizes the stream ID on the cell (it is either the ID of an open stream or the signaling (zero) ID), the OR processes the contents of the relay cell. Otherwise, it passes the decrypted relay cell along the circuit if the circuit continues, or drops the cell if it's the end of the circuit. [Getting an unrecognized relay cell at the end of the circuit must be allowed for now; we can reexamine this once we've designed full tcp-style close handshakes. -RD] Otherwise, if the data cell is coming from the OP edge of the circuit, the OP decrypts the length and payload fields with 3DES/OFB as follows: OP sends data cell to node R_M: For I=1...M, decrypt with Kf_I. Otherwise, if the data cell is arriving at the OP edge if the circuit, the OP encrypts the length and payload fields with 3DES/OFB as follows: OP receives data cell: For I=N...1, Encrypt with Kb_I. If the stream ID is a recognized stream for R_I, or if the stream ID is the signaling ID (zero), then stop and process the payload. For more information, see section 5 below. 5. Application connections and stream management 5.1. Streams Within a circuit, the OP and the exit node use the contents of RELAY packets to tunnel end-to-end commands and TCP connections ("Streams") across circuits. End-to-end commands can be initiated by either edge; streams are initiated by the OP. The first 8 bytes of each relay cell are reserved as follows: Relay command [1 byte] Stream ID [7 bytes] The recognized relay commands are: 1 -- RELAY_BEGIN 2 -- RELAY_DATA 3 -- RELAY_END 4 -- RELAY_CONNECTED 5 -- RELAY_SENDME 6 -- RELAY_EXTEND 7 -- RELAY_EXTENDED All RELAY cells pertaining to the same tunneled stream have the same stream ID. Stream ID's are chosen randomly by the OP. A stream ID is considered "recognized" on a circuit C by an OP or an OR if it already has an existing stream established on that circuit, or if the stream ID is equal to the signaling stream ID, which is all zero: [00 00 00 00 00 00 00] To create a new anonymized TCP connection, the OP sends a RELAY_BEGIN data cell with a payload encoding the address and port of the destination host. The stream ID is zero. The payload format is: ADDRESS | ':' | PORT | '\000' where ADDRESS may be a DNS hostname, or an IPv4 address in dotted-quad format; and where PORT is encoded in decimal. Upon receiving this packet, the exit node resolves the address as necessary, and opens a new TCP connection to the target port. If the address cannot be resolved, or a connection can't be established, the exit node replies with a RELAY_END cell. Otherwise, the exit node replies with a RELAY_CONNECTED cell. The OP waits for a RELAY_CONNECTED cell before sending any data. Once a connection has been established, the OP and exit node package stream data in RELAY_DATA cells, and upon receiving such cells, echo their contents to the corresponding TCP stream. [XXX Mention zlib encoding. -NM] When one side of the TCP stream is closed, the corresponding edge node sends a RELAY_END cell along the circuit; upon receiving a RELAY_END cell, the edge node closes the corresponding TCP stream. [This should probably become: When one side of the TCP stream is closed, the corresponding edge node sends a RELAY_END cell along the circuit; upon receiving a RELAY_END cell, the edge node closes its side of the corresponding TCP stream (by sending a FIN packet), but continues to accept and package incoming data until both sides of the TCP stream are closed. At that point, the edge node sends a second RELAY_END cell, and drops its record of the topic. -NM] For creation and handling of RELAY_EXTEND and RELAY_EXTENDED cells, see section 4. For creating and handling of RELAY_SENDME cells, see section 6. 6. Flow control 6.1. Link throttling As discussed above in section 2.1, ORs and OPs negotiate a maximum bandwidth upon startup. The communicants only read up to that number of bytes per second on average, though they may use mechanisms to handle spikes (eg token buckets). Communicants rely on TCP's default flow control to push back when they stop reading, so nodes that don't obey this bandwidth limit can't do too much damage. 6.2. Link padding Currently nodes are not required to do any sort of link padding or dummy traffic. Because strong attacks exist even with link padding, and because link padding greatly increases the bandwidth requirements for running a node, we plan to leave out link padding until this tradeoff is better understood. 6.3. Circuit flow control To control a circuit's bandwidth usage, each node keeps track of two 'windows', consisting of how many RELAY_DATA cells it is allowed to package for transmission, and how many RELAY_DATA cells it is willing to deliver to a stream outside the network. Each 'window' value is initially set to 500 data cells in each direction (cells that are not data cells do not affect the window). [Note: I'm not touching the rest of this section... it looks in the code as if RELAY_COMMAND_SENDME is now doing double duty for both stream flow control and circuit flow control. I thought we wanted two different notions of windows. -NM] [We do have two different 'levels' of windows. The relay sendme command is talking about a stream for non-zero stream id, and talking about the circuit for zero stream id. -RD] Each edge node on a circuit sends a SENDME cell (with length=100) every time it has received 100 data cells on the circuit. When a node receives a SENDME cell for a circuit, it increases the circuit's window in the corresponding direction (that is, for sending data cells back in the direction from which the sendme arrived) by the value of the cell's length field. If it's not an edge node, it passes an equivalent SENDME cell to the next node in the circuit. If the window value reaches 0 at the edge of a circuit, the OR stops reading from the edge connections. (It may finish processing what it's already read, and queue those cells for when a SENDME cell arrives.) Otherwise (when not at the edge of a circuit), if the window value is 0 and a data cell arrives, the node must tear down the circuit. 6.4. Topic flow control Edge nodes use RELAY_SENDME data cells to implement end-to-end flow control for individual connections across circuits. As with circuit flow control, edge nodes begin with a window of cells (500) per topic, and increment the window by a fixed value (50) upon receiving a RELAY_SENDME data cell. Edge nodes initiate TOPIC_SENDME data cells when both a) the window is <= 450, and b) there are less than ten cell payloads remaining to be flushed at that edge. 7. Directories and routers 7.1. Router descriptor format. (Unless otherwise noted, tokens on the same line are space-separated.) Router ::= Router-Line Public-Key Signing-Key? Exit-Policy NL Router-Line ::= "router" address ORPort OPPort APPort DirPort bandwidth NL Public-key ::= a public key in PEM format NL Signing-Key ::= "signing-key" NL signing key in PEM format NL Exit-Policy ::= Exit-Line* Exit-Line ::= ("accept"|"reject") string NL ORport ::= port where the router listens for other routers (speaking cells) OPPort ::= where the router listens for onion proxies (speaking cells) APPort ::= where the router listens for applications (speaking socks) DirPort ::= where the router listens for directory download requests bandwidth ::= maximum bandwidth, in bytes/s Example: router moria.mit.edu 9001 9011 9021 9031 100000 -----BEGIN RSA PUBLIC KEY----- MIGJAoGBAMBBuk1sYxEg5jLAJy86U3GGJ7EGMSV7yoA6mmcsEVU3pwTUrpbpCmwS 7BvovoY3z4zk63NZVBErgKQUDkn3pp8n83xZgEf4GI27gdWIIwaBjEimuJlEY+7K nZ7kVMRoiXCbjL6VAtNa4Zy1Af/GOm0iCIDpholeujQ95xew7rQnAgMA//8= -----END RSA PUBLIC KEY----- signing-key -----BEGIN RSA PUBLIC KEY----- 7BvovoY3z4zk63NZVBErgKQUDkn3pp8n83xZgEf4GI27gdWIIwaBjEimuJlEY+7K MIGJAoGBAMBBuk1sYxEg5jLAJy86U3GGJ7EGMSV7yoA6mmcsEVU3pwTUrpbpCmwS f/GOm0iCIDpholeujQ95xew7rnZ7kVMRoiXCbjL6VAtNa4Zy1AQnAgMA//8= -----END RSA PUBLIC KEY----- reject 18.0.0.0/24 Note: The extra newline at the end of the router block is intentional. 7.2. Directory format Directory ::= Directory-Header Directory-Router Router* Signature Directory-Header ::= "signed-directory" NL Software-Line NL Software-Line: "recommended-software" comma-separated-version-list Directory-Router ::= Router Signature ::= "directory-signature" NL "-----BEGIN SIGNATURE-----" NL Base-64-encoded-signature NL "-----END SIGNATURE-----" NL Note: The router block for the directory server must appear first. The signature is computed by computing the SHA-1 hash of the directory, from the characters "signed-directory", through the newline after "directory-signature". This digest is then padded with PKCS.1, and signed with the directory server's signing key.